In the field of implantable medical devices (IMD), electrical componentry is used to monitor the patient, disperse medications, and also to apply therapeutic electrical stimulation to tissue. These IMDs often have power requirements in excess of what can be provided by conventional batteries, thus rechargeable batteries are provided. To prevent the need for electrical wiring to protrude from a patient's skin, which presents an infection hazard, most implant rechargeable batteries are charged via an inductive field provided by an external charger to an implant charge receiving system included in the IMD. This implant charge receiving system allows the rechargeable battery of the IMD to be recharged through the patient's skin without physical contact between the implant and the external charging unit. In addition, some IMDs are continuously externally powered by inductive power fields provided by an external power unit, much like the external charger that is affixed to the patient whenever the device is used. These implants often do not include a rechargeable battery for powering the device. Cochlear devices in particular are often powered in this manner.
However, in both type of IMDs, with or without rechargeable batteries, inductive power fields are required to power the electrical componentry. Electrically driven implantable devices are used, for example, as neuro-stimulators including pain suppression, hearing aids (e.g. cochlear devices), cardiac pacemakers, and defibrillators. However, they may also be used for drug infusion and dispensing systems, nerve and bone growth stimulators, digestive track stimulators, artificial vision apparatus, artificial organs including artificial hearts, bladder stimulators, and for the purposes of implanted sensors that monitor but do not actively stimulate tissue. Additionally, IMDs have been used as combinations of the above listed devices, such as a combined cardiac pacemaker and cardiac defibrillator. Thus, the electrical componentry may provide therapeutic electrical stimulation of tissue, monitoring of the patient, medication dispensing, and other medical purposes. Further, the electrical componentry may also include circuitry for monitoring the IMD itself and for communicating with external programmers, device controllers, and patient information gathering systems.
As a result of the proliferation of electrically powered IMDs into new areas of medical treatment, electronic components have been upgraded and perform ever more complicated monitoring/diagnosis and therapeutic electrical stimulation using smaller electrical circuitry while rechargeable batteries have increased in life span and energy density and decreased their recharging times. However, despite all of the improvements in electrically powered IMDs since the invention of the original pacemaker in the late 1950s, transdermal inductive powering, for charging of rechargeable batteries or direct powering of an implant, is now the medical standard for powering of many implants. However, despite the fact that new rechargeable battery technologies allow for fast recharging of implanted batteries, for example newer lithium ion chemistries may be recharged in an hour, whereas IMDs often require longer charging times. These longer charging times are necessitated, not by battery chemistries that prevent fast recharges, but due to the heat generated during transdermal inductive charging or powering of the device. For example, an inductive power field, from an external charger, may cause substantial eddy currents on the housing of the implant, which cause significant heat to build up in the implant. Furthermore, substantial heat is generated by the recharging circuitry in the implant during the process of converting the inductive power field into a useable current for the rest of the electrical circuitry engaged in, for example therapeutic electrical stimulus, patient monitoring, battery charging, and telemetry with external devices. Thus, heat generation increases recharging times for electrically powered implants, inconveniencing patients. One method proposed for reducing charging times is to include a fan with the external charger, U.S. Pat. No. 5,991,665 to Wang et al. However, this solution primarily cools the skin and does not increase the circulation of fluids within the body that come in contact with the implant's housing, which is used as a heat sink.
Additionally, heat generating electrical circuitry, in particular recharging circuitry, is usually located in limited portions of the implantable device causing hotspots on areas of the implant housing located near the heat generating componentry. Excessive heating on the housing external surfaces may cause necrosis (tissue death) in the areas exposed to the hotspots. Accordingly, industry standards allow the external portion of the implant housing, that is exposed to the patients tissue, to be only 3 degrees higher in Celsius temperature than the surrounding tissue, which is about 37 degrees Celsius. Thus, inductive power fields currently must be lowered to reduce temperatures which results in an increase in charging times. There is a need for a way to evenly disperse heat generated in electrically powered IMDs over the entire housing to prevent hotspots. This effectively increases the usable surface area of the heat sink, which is the external portion of the implant housing. Thus, dispersing heat throughout the implant housing would allow for the use of stronger inductive power fields which decreases recharging times and/or increases the amount of power supplied to implant.